WASHINGTON—Nationals outfielder Bryce Harper is not convinced he can begin a rehab assignment as quickly as his manager believes.

Davey Johnson had said Friday that he expects Harper to start a rehab stint Tuesday at Class A Potomac. Harper, who has been sidelined since May 26 with left knee bursitis, said he thought he needed an extra day or two.

"It just depends how I feel today and tomorrow," Harper said before Saturday's game against the Colorado Rockies. "Monday is an off day, which is good. Tuesday, that's kind of early. I'm thinking Wednesday or Thursday, maybe."

Informed later of Harper's comments, Johnson said, "I'm always trying to do what's best for the player. But at the same time, it's my job to know when they're ready and when they're not."

Harper ran in the outfield and took swings in the indoor batting cage the previous two days. Entering Saturday, he had missed 29 of the Nationals' 73 games.